Multilayer hollow fibre membranes are well-known in the art. They usually consist of a supporting layer and a separation layer which can be made of different materials (asymmetric composite membranes) or of essentially the same materials (asymmetric integrally skinned membranes). In both cases, the supporting layer and separating layer posses a different morphology. However, in the case of asymmetric composite membranes, the supporting layer and the separating layer are bounded physically which is not favourable. In the case of asymmetric integrally skinned membrane, fabrication of the separating layer on the lumen side is not always possible.
WO 2005/082502, incorporated by reference, discloses a high-flux dialysis membrane having an asymmetric structure which is made by feeding a spinning solution comprising a hydrophobic polymer and a solvent through the annular orifice of a hollow fibre die, co-extruding a coagulant composition comprising a solvent, a non-solvent for the hydrophobic polymer, and a polyelectrolyte, and passing the obtained hollow fibre membrane through a coagulation bath, wherein the polyelectrolyte is precipitated on the lumen-side of the hollow fibre and becomes physically bound to the separating layer. The spinning solution may further comprise a hydrophilic polymer to increase the viscosity of the spinning solution. After formation of the hollow fibre membrane, the hydrophilic polymer may be cross-linked after formation of the hollow fibre membrane. Hence, WO 2005/082502 discloses a two-step process.
WO 2007/125367 and WO 2008/138078, both incorporated by reference, disclose multi-step processes for manufacturing asymmetric membranes made form polyimides. The process involves casting a film of a polyimide solution onto a substrate, immersing the film into a coagulation medium to form the membrane and treating the formed membrane with an amine.
WO 2009/088978, incorporated by reference, discloses a process for preparing mono-esterified polyimides and the use thereof in the manufacture of hollow fibre membranes comprising cross-linked polyimides, wherein the cross-linking step comprises contacting the mono-esterified polyimides with a cross-linking agent, preferably a diol. Hence, WO 2009/088978 discloses a two-step process.
WO 2007/007051, incorporated by reference, discloses triple-orifice and quadruple-orifice hollow fibre dies. Such dies are also known from S.-G. Li et al., J. Membrane Sci. 94, 329-340, 1994, and from WO 93/12868, both incorporated herein by reference.
It is an object of the present invention to provide a process that enables the manufacture of hollow fibre membranes having a supporting layer and an inner (i.e. on the lumen-facing side of the supporting layer) or an outer separating layer in a single step.